A power supply usually comprises a main power system and a standby power system. By a main transformer, the main power system is divided into a power driver unit at the primary side and a rectification unit at the secondary side. The rear end of the rectification unit is coupled to a voltage feedback unit 6 to correct the working period of the power driver unit. Refer to FIG. 1 for the circuit architecture of a conventional power supply. In the conventional power supply shown in FIG. 1, the power driver unit has a primary driver unit SW1, a secondary driver unit SW2 and a main transformer TX1. The rectification unit utilizes a self-excitation coil Lo to generate a self-excitation phenomenon. The charge/discharge of the self-excitation coil Lo is used to control the turn-on of a first rectifier switch and a second rectifier switch. For example, a R.O.C. patent application No.093203328 “Forward-Type Power Supply with Self-Excitation Synchronous Rectification Circuit” has a self-excitation coil and a self-excitation driver for the self-excitation coil inside the rectification unit at the secondary side of its main transformer. Further, four US patent Publication Nos. 20020097588, 20020196002, 20050047177 and 20060018133 also utilize self-excitation technologies for power transformation. Therefore, self-excitation synchronous rectification technologies have been widely used in forward-type power transformation devices. The sensed voltage of the main transformer is directly or indirectly used to drive the synchronous rectifier switch elements and achieve synchronous self-excitation in all the existing patents of self-excitation synchronous rectification technologies. Refer to FIG. 2 for the waveforms of the conventional power supply circuit shown in FIG. 1. In conventional power supplies, when the load varies abruptly, the self-excitation frequency will be out of control, which will results in abnormal voltage and damage. The conventional self-excitation technologies cannot achieve the external synchronous function via that the self-excitation coil senses the activities of the power driver unit at the primary side of the main transformer. Thus, the timings of the first and second rectifier switches cannot separate, and overlap loss occurs. When energy is recycled during the zero-load stage, the operation of the primary driver switch of the power driver unit stops; thus, an abnormally low self-excitation frequency and a surge voltage 5 appear. Besides, during the underload-shutdown stage, the secondary side of the main transformer still has unreleased energy with the power driver unit at the primary side having been turned off, and the voltage is thus out of control. Therefore, in the conventional self-excitation technologies, the self-excitation frequency of the rectification unit is likely to get out of control during the underload-shutdown stage or the zero-load stage. Obviously, the art needs a mechanism to regulate the self-excitation frequency.